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| United States Patent |
6,020,963
|
|
DiMarzio
|
February 1, 2000
|
Optical quadrature Interferometer
Abstract
An optical quadrature interferometer is presented. The optical quadrature
interferometer uses a different state of polarization in each of two arms
of the interferometer. A light beam is split into two beams by a
beamsplitter, each beam directed to a respective arm of the
interferometer. In one arm, the measurement arm, the light beam is
directed through a linear polarizer and a quarter wave plate to produce
circularly polarized light, and then to a target being measured. In the
other arm, the reference arm, the light beam is not subject to any change
in polarization. After the light beams have traversed their respective
arms, the light beams are combined by a recombining beamsplitter. As such,
upon the beams of each arm being recombined, a polarizing element is used
to separate the combined light beam into two separate fields which are in
quadrature with each other. An image processing algorithm can then obtain
the in-phase and quadrature components of the signal in order to construct
an image of the target based on the magnitude and phase of the recombined
light beam. The system may further be used for lensless imaging.
| Inventors:
|
DiMarzio; Charles A. (Cambridge, MA)
|
| Assignee:
|
Northeastern University (Boston, MA)
|
| Appl. No.:
|
985691 |
| Filed:
|
December 5, 1997 |
| Current U.S. Class: |
356/491 |
| Intern'l Class: |
G01B 009/02 |
| Field of Search: |
356/351,345
|
References Cited
U.S. Patent Documents
| 3620589 | Nov., 1971 | Dudderar | 356/351.
|
| 4183671 | Jan., 1980 | Jacobson | 356/354.
|
| 4418981 | Dec., 1983 | Stowe | 350/96.
|
| 4426620 | Jan., 1984 | Buchenauer | 324/83.
|
| 4514054 | Apr., 1985 | Stowe | 350/96.
|
| 4789240 | Dec., 1988 | Bush | 356/345.
|
| 4899042 | Feb., 1990 | Falk et al. | 250/227.
|
| 5101450 | Mar., 1992 | Olshansky | 385/3.
|
| 5172186 | Dec., 1992 | Hosoe | 356/351.
|
| 5200795 | Apr., 1993 | Kim et al. | 356/345.
|
| 5289256 | Feb., 1994 | Gramling | 356/345.
|
| 5367175 | Nov., 1994 | Bobb | 250/577.
|
| 5619325 | Apr., 1997 | Yoshida | 356/351.
|
Primary Examiner: Kim; Robert H.
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin & Hayes LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of U.S. application Ser. No.
08/658,087, filed Jun. 4, 1996 now U.S. Pat. No. 5,883,717 issued Mar. 16,
1999. This application further claims priority under 35 U.S.C.
.sctn.119(e) to provisional patent application Ser. No. 60/032,923 filed
Dec. 6, 1996; the disclosure of which is incorporated herein by reference.
Claims
We claim:
1. An optical quadrature interferometer comprising:
a light source providing a beam of light;
a first beamsplitter oriented at an angle with respect to said light
source, said first beamsplitter receiving said beam of light and splitting
said beam of light into a first secondary light beam and a second
secondary light beam;
a recombining beamsplitter;
a first reflective element oriented such that said first reflective element
receives said first secondary light beam and reflects said first secondary
light beam along a different axis to said recombining beamsplitter;
a second reflective element receiving said secondary light beam and
oriented to direct said secondary light beam along a different axis;
a quarter wave plate receiving said second secondary light beam from said
second reflective element, said quarter wave plate providing a 90.degree.
phase shift between vertically and horizontally polarized components of
the beam resulting in a circularly polarized light beam;
a target receiving said circularly polarized light beam, said target
causing a change to said circularly polarized light beam resulting in a
measurement light beam;
said recombining beamsplitter combining said measurement light beam with
said first secondary light beam to provide a first resultant light beam
and a second resultant light beam, said first secondary light beam having
approximately the same frequency as said beam of light;
a first polarizing beamsplitter receiving said first resultant light beam
and splitting said first resultant light beam into a first secondary
resultant light beam and a second secondary resultant light beam;
a second polarizing beamsplitter receiving said second resultant light beam
and splitting said second resultant light beam into a third secondary
resultant light beam and a fourth secondary resultant light beam; and
at least one imaging system for displaying data from said first secondary
resultant light beam, said second secondary resultant light beam, said
third secondary resultant light beam, and said fourth secondary resultant
light beam.
2. The optical quadrature interferometer of claim 1 wherein the change to
said circularly polarized light beam by said target comprises a phase
shift.
3. The optical quadrature interferometer of claim 1 wherein the change to
said circularly polarized light beam by said target comprises a change in
amplitude.
4. The optical quadrature interferometer of claim 1 wherein said imaging
system comprises a scattering screen followed by a charge coupled device
camera and a computer.
5. The optical quadrature interferometer of claim 1 wherein said imaging
system comprises photographic film and an optical reconstruction system.
6. The optical quadrature interferometer of claim 1 wherein said light
source comprises a laser.
7. The optical quadrature interferometer of claim 1 wherein said light
source comprises a light source having a short coherence length.
8. The optical quadrature interferometer of claim 6 wherein said light
source having a short coherence length is chosen from a group comprising a
light emitting diode, a superluminescent diode, a dye laser, a gas
discharge lamp, a tungsten filament lamp, and a mercury arc lamp.
9. The optical quadrature interferometer of claim 1 wherein said at least
one imaging system comprises a first imaging system for displaying data
from said first secondary resultant light beam; a second imaging system
for displaying data from said second secondary resultant light beam; a
third imaging system for displaying data from said third secondary
resultant light beam; and a fourth imaging system for displaying data from
said fourth secondary resultant light beam.
10. An optical quadrature interferometer comprising:
a light source providing a beam of light;
a first beamsplitter oriented at an angle with respect to said light
source, said first beamsplitter receiving said beam of light and splitting
said beam of light into a first secondary light beam and a second
secondary light beam;
a recombining beamsplitter;
a first reflective element oriented such that said first reflective element
receives said first secondary light beam and reflects a portion of said
first secondary light beam along a different axis to said recombining
beamsplitter;
a second reflective element receiving said secondary light beam and
oriented to direct said secondary light beam along a different axis;
a quarter wave plate receiving said second secondary light beam from said
second reflective element, said quarter wave plate providing a 90.degree.
phase shift between vertically and horizontally polarized components of
the beam resulting in a circularly polarized light beam;
a target receiving said a portion of the first secondary light beam not
reflected by said first reflective element, said target causing a change
to the portion of the first secondary light beam received by said target
resulting in a measurement light beam;
said recombining beamsplitter combining said measurement light beam with
said first secondary light beam to provide a first resultant light beam
and a second resultant light beam, said first secondary light beam having
approximately the same frequency as said beam of light;
a first polarizing beamsplitter receiving said first resultant light beam
and splitting said first resultant light beam into a first secondary
resultant light beam and a second secondary resultant light beam;
a second polarizing beamsplitter receiving said second resultant light beam
and splitting said second resultant light beam into a third secondary
resultant light beam and a fourth secondary resultant light beam; and
at least one imaging system for displaying data from said first secondary
resultant light beam, said second secondary resultant light beam, said
third secondary resultant light beam, and said fourth secondary resultant
light beam.
11. The optical quadrature interferometer of claim 10 wherein the change to
said circularly polarized light beam by said target comprises a phase
shift.
12. The optical quadrature interferometer of claim 10 wherein the change to
said circularly polarized light beam by said target comprises a change in
amplitude.
13. The optical quadrature interferometer of claim 10 wherein said imaging
system comprises a scattering screen followed by a charge coupled device
camera and a computer.
14. The optical quadrature interferometer of claim 10 wherein said imaging
system comprises photographic film and an optical reconstruction system.
15. The optical quadrature interferometer of claim 10 wherein said light
source comprises a laser.
16. The optical quadrature interferometer of claim 10 wherein said light
source comprises a light source having a short coherence length.
17. The optical quadrature interferometer of claim 16 wherein said light
source having a short coherence length is chosen from a group comprising a
light emitting diode, a superluminescent diode, a dye laser, a gas
discharge lamp, a tungsten filament lamp, and a mercury arc lamp.
18. The optical quadrature interferometer of claim 10 wherein said at least
one imaging system comprises a first imaging system for displaying data
from said first secondary resultant light beam; a second imaging system
for displaying data from said second secondary resultant light beam; a
third imaging system for displaying data from said third secondary
resultant light beam; and a fourth imaging system for displaying data from
said fourth secondary resultant light beam.
Description
BACKGROUND OF THE INVENTION
Mach-Zender interferometers and Michelson interferometers are known in the
art. The interferometers typically include a beamsplitter which divides a
signal from a light source into two separate signals which are in-phase
with one another. One signal is sent through a reference arm which may
include a compensation element. The other signal is sent through a
measurement arm, in which the optical signal is exposed to some change in
amplitude and phase. In the Mach-Zender interferometer, after the signals
have propagated through their respective reference and measurement arms,
the signals are combined, whereas in the Michelson interferometer, after
the signals have propagated through their respective arms they are
reflected back through the arms again where they are then combined. The
combined beams produce an interference pattern with bright and dark
regions indicative of the phase of the signal beam relative to the
reference. The dependence of the brightness of the interference pattern on
the relative phase and amplitude is complicated. The interferometers are
sensitive to noise, and the algorithms involved in performing the phase
retrieval and comparisons are cumbersome, involving lengthy calculations
and introducing errors.
BRIEF SUMMARY OF THE INVENTION
An optical quadrature interferometer according to the present invention
includes a light source which may be polarized or an unpolarized light
source and a polarizer, and a beamsplitter which splits the light provided
by the light source into two signals which are in a defined phase
relationship with each other. In one path a quarter wave plate is used to
convert the polarization of the light to circular. This circularly
polarized light can be represented by equal amounts of vertically and
horizontally polarized light with a phase difference of 90.degree. between
them. The circularly polarized signal is then transmitted through a target
and the resulting signal is combined with the reference signal. The
combined signal is then directed through a polarizer which can switch
between vertical polarization and horizontal polarization or alternatively
through a polarizing beamsplitter. The combined signal can thus be
separated into two separate fields which are in quadrature with each
other. An image processing algorithm can then obtain the in-phase and
quadrature information and provide detailed images of the amplitude and
phase of the target.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The invention will be more fully understood from the following detailed
description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a diagram of a prior art Mach-Zender type interferometer;
FIG. 2 is a diagram of a first implementation of an optical quadrature
interferometer;
FIG. 2A is a diagram of the optical quadrature interferometer of FIG. 2
including an additional imaging system;
FIG. 2B is a diagram of the optical quadrature interferometer of FIG. 2A
including an additional imaging system channel;
FIG. 3 is a diagram of a prior art Michelson type interferometer;
FIG. 4 is a diagram of an additional implementation of an optical
quadrature interferometer;
FIG. 4A is a diagram of the optical quadrature interferometer of FIG. 4
including an additional imaging system;
FIG. 5 is a diagram of an optical quadrature interferometer including a
double balanced imaging system; and
FIG. 5A is a diagram of an additional embodiment of an optical quadrature
interferometer including a double balanced imaging system.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a prior art Mach-Zender interferometer 10. This interferometer
comprises a light source 20, such as a laser, providing a beam of light 80
which is directed to a beamsplitter 30. The beamsplitter 30 splits the
beam of light 80 into two secondary beams 82 and 84 which are in phase
with each other. The beamsplitter 30 is angled with respect to light
source 20 such that the reflected beam is directed along an axis which is
different than that of light beam 80. Although angles of approximately
ninety degrees are shown in the Figures, it should be appreciated that any
angle could be used. The first secondary beam 82 is the portion that
passes through the beamsplitter 30. This first secondary beam 82 is
directed to a first mirror 40, which is angled to reflect the first
secondary light beam 82 along a different axis to a recombining
beamsplitter 60. This path, from beamsplitter 30 to first mirror 40 then
to recombining beamsplitter 60 comprises the reference arm 94, and
provides for a beam unaffected by the target 90.
Second secondary beam 84 is the light beam reflected by the beamsplitter 30
which is angled such that the reflected light is directed to a second
mirror 50. Second mirror 50 is angled such that the second secondary beam
84 is directed through a target 90. The second secondary beam 84 is
subjected to a phase change by passing through target 90 resulting in
measurement beam 85. This path from beamsplitter 30, to mirror 50, through
target 90, to recombining beamsplitter 60 comprises the measurement arm
92.
First secondary beam 82 and measurement beam 85 are combined by recombining
beamsplitter 60 to provide resulting beam 86. Recombining beamsplitter 60
is angled such that resulting beam 86 is directed to detector 70. Any
difference in phase between first secondary beam 82 and measurement beam
85 can be determined by analysis of resulting beam 86 by detector 70. A
phase retrieval algorithm is used to determine the density and optical
thickness or other characteristics of the target 90.
Referring now to FIG. 2 a first implementation of an optical quadrature
interferometer 100 is shown. The optical quadrature interferometer 100 is
based on a Mach-Zender interferometer and comprises a light source 20,
such as a laser, providing a beam of light 80 which is directed to a
beamsplitter 30. When a light source 20 which is not well polarized is
used, a polarizer 150 which polarizes the light at 45.degree. to the
vertical, may be inserted between the light source 20 and the beamsplitter
30. Other angles are also possible, provided that a corresponding change
is made at the output. If a well polarized light source is used, polarizer
150 is not necessary. The beamsplitter 30 splits the beam of light 80 into
two secondary beams 82 and 84 which are in phase with each other. The
beamsplitter 30 is angled such that a portion of the beam 84 is reflected
along a different axis than that of beam 80, and a portion of the beam 82
passes through the beamsplitter 30.
The portion of light reflected by beamsplitter 30 is the second secondary
light beam 84. This beam is directed through a quarter wave plate 120 by
second reflective element 50. The quarter wave plate 120 provides for a
relative phase shift of .DELTA..phi.=.pi./2 radians which is equivalent to
a phase shift of 90.degree. between horizontal and vertical components of
phase shifted beam 81. Phase shifted beam 81 is then directed through
target 90. As phase shifted beam 81 passes through target 90, phase
shifted beam 81 may undergo a change in phase, amplitude or both,
resulting in measurement beam 85. This path comprises the measurement arm
92.
The first secondary beam 82 is directed to a reference 110 which alters the
phase of light beam 83 to nearly match that of beam 84. For example, if
the target being measured was a test-tube of water containing a fiber to
be measured, the compensation element would be a test-tube of water
without the fiber, such that any phase or amplitude change resulting from
the test-tube and water in the measurement arm is compensated for in the
reference arm. Accordingly, the interference pattern will not contain high
spatial-frequency components which would require very good spatial
resolution. Compensated light beam 83 is directed to first reflective
element 40. First reflective element 40 is angled to reflect the
compensated beam 83 to an recombining beamsplitter 60. This path, from
beamsplitter 30 through reference element 110, to first reflective element
40 and to recombining beamsplitter 60 comprises the reference arm 94.
Compensated light beam 83 and measurement beam 85 are combined by
recombining beamsplitter 60 to provide resulting beam 86. Recombining
beamsplitter 60 is angled such that resulting beam 86 is directed along a
different axis than compensated light beam 83. Resulting beam 86 is
directed through polarizer 140. Polarizer 140 is rotated between two
positions, a first position where polarizer 140 is a horizontal polarizer,
and a second position where polarizer 140 is a vertical polarizer.
Polarizer 140 is used to separate the mixed polarization field of
resulting beam 86 into two separate fields which are in quadrature with
each other by rotating polarizer 140 between it's two positions. The beam
resulting from polarizer 140 is directed to imaging system 75 which
utilizes an image processing algorithm to obtain the in-phase and
quadrature components of the signal, thereby providing interpretation of
the data using both the magnitude and phase information.
The imaging system 75 may comprise a scattering screen such as ground glass
or milk glass which is followed by a charge coupled device (CCD) camera or
other television camera, and a computer system including a device for
storing and retrieving image frames from the CCD camera. The imaging
system may also comprise photographic film and an optical reconstruction
system for producing holographic images from the processed film, or direct
display on a CCD.
Referring now to FIG. 2A, an optical quadrature interferometer 150 is
shown. This optical quadrature interferometer is similar to the one shown
in FIG. 2 except that recombining beamsplitter 60 has been replaced with
polarizing beamsplitter 65, polarizer 140 has been removed, and a dual
channel imaging system 75 and 75' are implemented to provide
interpretation of the data using both the phase and magnitude information.
Resultant beam 86 is directed to polarizing beamsplitter 65 which provides
for two polarized output beams 87 and 88. Each polarized output beam 87
and 88 is provided to a respective imaging system 75 and 75'.
Referring now to FIG. 2B, an optical quadrature interferometer 160 is
shown. This optical quadrature interferometer is similar to the one shown
in FIG. 2A and includes an additional polarizing beamsplitter 66, and an
additional dual channel imaging system 76 and 76'. A second resultant beam
89 is also provided by recombining beamsplitter 60. While this second
resultant beam 89 is ignored in other embodiments, it is utilized in this
embodiment. Second resultant beam 89 is directed to second polarizing
beamsplitter 66 which provides for two polarized output beams 91 and 92.
Each polarized output beam 91 and 92 are provided to a respective imaging
system 76 and 76', thus providing for a four channel imaging system.
Referring now to FIG. 3, a prior art Michelson interferometer 200 is shown.
A light source 20, such as a laser, provides a beam of light 80 which is
directed to a beamsplitter 30. The beamsplitter 30 splits the beam of
light 80 into two secondary beams 82 and 84 which are in phase with each
other. The first secondary beam 82 is the portion that passes through the
beamsplitter 30. This light beam is directed to a first mirror 40, which
is angled to deflect the first secondary light beam along the same axis
back to the beamsplitter 30. This path is the reference arm 96, and
provides for a beam unaffected by a target.
Second secondary beam 84 is the light beam reflected by the beamsplitter
30. Beamsplitter 30 is angled such that the reflected light is directed to
a target 90 where secondary beam 84 undergoes a phase shift upon passing
through target 90. The beam exiting target 90 is directed to second mirror
50. Second mirror 50 is angled so that the second secondary beam is
directed back through the target 90. The second secondary beam is
subjected to a second phase change by target 90. First secondary beam 82
and second secondary beam 84 are combined by beamsplitter 30 to provide
resulting beam 86. Any difference in phase between first secondary beam 82
and second secondary beam 84 can be determined by analysis of resultant
beam 86 by detector 70. A phase retrieval algorithm is used to determine
the density, clarity or other characteristics of the target 90.
FIG. 4 shows an embodiment of an optical quadrature interferometer 250
based on a Michelson interferometer. The optical quadrature interferometer
250 comprises a light source 20, such as a laser, providing a beam of
light 80 which is directed to a beamsplitter 30. An optional polarizer 150
which polarizes the light at 45.degree. (or other suitable angle) to the
vertical may be used if the light source 20 is not well polarized. The
beamsplitter 30 splits the beam of light 80 into two secondary beams 82
and 84 which are in a known phase relationship with each other. The first
secondary beam 82 is directed to a reference 110 resulting in compensated
light beam 83. Compensated light beam 83 is then directed to a first
reflective element 40. First reflective element 40 is oriented such that
the compensated light beam is reflected back along the same axis, through
reference 110, and to beamsplitter 30.
In the measurement arm 98 of the optical quadrature interferometer 250, the
second secondary light beam 84 is directed through a one-eighth wave plate
130. The one-eighth wave plate 130 provides for a relative phase shift of
.DELTA..phi.=.pi./4 radians which is equivalent to a phase shift of
45.degree. between the vertical and horizontal components of the beam
entering the one-eighth wave plate 130. Beam 81 is then directed through
target 90, where it undergoes a change in phase, amplitude or both;
resulting in measurement beam 85. Measurement beam 85 is then directed to
second reflective element 50 which is oriented so that beam 85 is
reflected back through target 90, then through one-eighth wave plate 130,
where the beam is phase shifted another 45.degree., resulting in the beam
being shifted a total of 90.degree. between the vertical and horizontal
polarization components of the beam, not including any phase shift as a
result of the beam passing through the target 90.
Compensated light beam 83 and measurement beam 85 are combined by
beamsplitter 30 to provide resulting beam 86. Resulting beam 86 is then
directed to polarizer 140 which is used to separate the mixed polarization
field into two separate fields which are in quadrature with each other by
rotating polarizer 140. Polarizer 140 is rotated between two positions, a
first position where polarizer 140 is a horizontal polarizer, and a second
position where polarizer 140 is a vertical polarizer. Polarizer 140 is
used to separate the mixed polarization field of resulting beam 86 into
two separate fields which are in quadrature with each other by rotating
polarizer 140 between it's two positions. The beam resulting from
polarizer 140 is directed to imaging system 75 which uses an image
processing algorithm to obtain the in-phase and quadrature components of
the signal, thereby providing interpretation of the data using both the
magnitude and phase information.
Referring now to FIG. 4A, an optical quadrature interferometer 260 is
shown. This optical quadrature interferometer is similar to the one shown
in FIG. 4 except that polarizer 140 has been replaced with polarizing
beamsplitter 65, and a dual channel imaging system 75 and 75' is
implemented to provide interpretation of the data using both the phase and
magnitude information. Resultant beam 86 is directed to polarizing
beamsplitter 65 which provides for two polarized output beams 87 and 88.
Each polarized output beam 87 and 88 is provided to a respective imaging
system 75 and 75'.
Additional embodiments may include the use of any type of laser as the
light source 20; a light source 20 having a short coherence length, such
as a light emitting diode (LED); a superluminescent diode; a dye laser; a
gas-discharge lamp; a tungsten filament lamp; a mercury arc lamp or
similar type light source. A collimator or focussing system may be
installed within the interferometer to change the intensity of the light
beam. The imaging system 75 may include any type of imaging system,
including photographic film and an optical reconstruction system to
produce a holographic image of the medium being measured. In an alternate
embodiment the image may be recovered by an imaging system utilizing
diffraction tomography. Power reducing filters may be implemented in
either arm of the interferometer when using cameras of limited irradiance
resolution, such as 8-bit CCD cameras, as part of the imaging system.
By use of polarization in the optical quadrature interferometer to obtain
the in-phase and quadrature information, the interferometry is more
accurate and does not require phase retrieval algorithms. Additionally,
the optical quadrature interferometer can be utilized in performing
optical coherence tomography faster and at less expense, and holograms can
be produced without ghost images. The optical quadrature interferometer
may be used in applications such as non-destructive testing of optical
fibers; transillumination imaging through turbid media, medical imaging
through tissue such as performing mammography at optical wavelengths; and
for vibration analysis.
A further embodiment, shown in FIGS. 5 and 5A, provides additional
features. In this embodiment an image of an object can be acquired without
a lens. The complex signal, at the CCD plane, may be back-propagated to an
arbitrary plane by the Fresnel-Kirchoff Integral to produce an image of a
slice of the object at that distance. The process may be repeated an
arbitrary number of times using different distances to provide a set of
images equivalent to those obtained in a conventional imaging system by
varying the focus mechanically. This results in shorter measurement time
and reduced exposure of the target to light, as all the data are collected
simultaneously. This is important for example, in imaging of transient
phenomena and in biological imaging.
A scanning confocal laser microscope provides useful information about
diffusive media such as biological tissue by focussing a source and
receiver at a common point. In the present invention, one pixel of the CCD
is equivalent to the receiver in the scanning confocal laser microscope,
and the other pixels provide additional information, making it possible to
provide more quantitative information about the object and reducing the
need for multiple views.
As shown in FIG. 2, the target 90 maybe removed and a reference picture
taken. In the absence of aberrations or misalignment, no fringes will be
seen. Thus, the fringes which are present are the result of an imperfect
system. Taking an image with the object in place, and dividing the complex
amplitude at each pixel in this image by that in the same pixel of the
reference image, the effect of the object alone maybe determined. Thus,
low-quality optics may be used in the system without degrading the image.
This concept also works in the configuration shown in FIG. 4.
In the configuration of FIG. 4A, the output from 75 is subtracted from 76,
pixel-by-pixel, and the output from 75' is subtracted from 76', with the
result multiplied by the square root of negative one, resulting in a
complex image with common mode noise and the non-interferometric part of
the image both being rejected. Appropriate filters in hardware or scale
factors in software may be required to make all signals equal in strength,
i.e. to compensate for unequal beamsplitting. Furthermore, it is possible
to arrange the beams as shown in FIG. 5 so that they are incident on
different quadrants of a single CCD.
The configuration shown in FIG. 5A, shows the same situation with a
modified Mach-Zehnder interferometer providing the same capability as in
FIG. 4, but for reflective targets.
Referring now to FIG. 5 a further implementation of an optical quadrature
interferometer 300 is shown. The optical quadrature interferometer 300
comprises a light source 20, such as a laser, providing a beam of light 80
which is directed to a beamsplitter 30. When a light source 20 which is
not well polarized is used, a polarizer 150 which polarizes the light at
45.degree. to the vertical, may be inserted between the light source 20
and the beamsplitter 30. Other angles are also possible, provided that a
corresponding change is made at the output. If a well polarized light
source is used, polarizer 150 is not necessary. The beamsplitter 30 splits
the beam of light 80 into two secondary beams 82 and 84 which are in phase
with each other. The beamsplitter 30 is angled such that a portion of the
beam 84 is reflected along a different axis than that of beam 80, and a
portion of the beam 82 passes through the beamsplitter 30.
The portion of light reflected by beamsplitter 30 is the second secondary
light beam 84. This beam is directed through a quarter wave plate 120 by
second reflective element 50. The quarter wave plate 120 provides for a
relative phase shift of .DELTA..phi.=.pi./2 radians which is equivalent to
a phase shift of 90.degree. between horizontal and vertical components of
phase shifted beam 81.
The first secondary beam 82 is directed to a target 90 which alters the
phase of light beam 82 resulting in measurement beam 85 which is directed
to first reflective element 40. First reflective element 40 is angled to
reflect the measurement beam 85 to an recombining beamsplitter 60.
Measurement beam 85 and phase shifted beam 81 are combined by recombining
beamsplitter 60 to provide first resultant beam 86 and second resultant
beam 87. First resultant beam 86 is directed through first polarizing
beamsplitter 65. Polarizing beamsplitter 65 is used to separate resultant
beam 86 into two separate beams 67 and 68. The beams 67 and 68 resulting
from polarizing beamspllitter 65 are directed to an imaging system (not
shown) which utilizes an image processing algorithm to provide
interpretation of the data. Second polarizing beamsplitter 66 is used to
separate the second resultant beam 87 into two separate beams 61 and 62.
The beams 61 and 62 resulting from second polarizing beamsplitter 66 are
directed to the imaging system (not shown) which utilizes an image
processing algorithm to provide interpretation of the data.
Referring now to FIG. 5A, a further embodiment of an optical quadrature
interferometer 350 is shown. This optical quadrature interferometer is
similar to the one shown in FIG. 5 except that only the light reflected by
the target 90 is recombined with the reference beam 81.
Considerable simplification of the processing and reduced processing time
can be realized by using both outputs of the interferometer. The use of
both outputs as a double balanced system is used for common mode
rejection. The phase of the signal relative to the reference differs by
180 degrees between the two outputs, so the difference between the two
outputs is:
.vertline.E.sub.sig +E.sub.ref .vertline..sup.2 -.vertline.E.sub.sig
-E.sub.ref .vertline..sup.2 =4RE*.sub.sig E*.sub.ref.
The above equation assumes each path has an equal amplitude, so some
attenuation may be required in one path in order to provide the equal
amplitude. The I and Q phases are needed on both outputs, and can be
obtained as shown in FIGS. 5 and 5A. Each of the four outputs may be
directed to a separate imaging device or to different parts of a single
array.
Having described preferred embodiments of the invention it will now become
apparent to those of ordinary skill in the art that other embodiments
incorporating these concepts may be used. Accordingly, it is submitted
that the invention should not be limited to the described embodiments but
rather should be limited only by the spirit and scope of the appended
claims.
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